APOE and Alzheimer disease: Dopamine production has been detected in staphylococci in the human intestine, which can take up the precursor l-DOPA and convert it into dopamine by SadA expressed by these bacteria [32].Significant differences in the gut microbial metabolism of tryptophan, SCFAs, and lithocholic acid were found among AD, amnestic MCI, and healthy control groups, and a metabolite of tryptophan, indole-3-pyruvic acid, is identified as a marker for AD diagnosis [16]. The abundances of Prevotellaceae and Ruminococcaceae and butyrate-producing bacteria are associated with APOE genotypes [18].